I used to live in Slough and I own a tandem. Generally train operators do not allow Tandems, but worth checking with them. Trains for Slough go from Paddington which is 3.5 miles away

There is a cycle track alongside the grand union canal, that goes from Regent park (about a mile from St Pancras) all the way into Slough. If the Tandem has robust tyres it is a good option (about 1h30min ride)

worth asking thameslink and SW if they can put it on their trains (it would need to be off peak) thameslink down to Wimbledon and then SW trains Wimbledon to Windsor and Eton (might need to change at Clapham, I cant remember) OR cycle to Paddington and get the train from Paddington to Windsor.

might be worth them checking with the organisers to see if there is any transport being laid on for athletes arriving by train into central London?
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